The invention generally relates to coating methods and coated articles. More particularly, the invention relates to combustion-based thermal spray coating methods and coated articles.
Cermet coatings may be used in various components to impart wear resistance. A thermal spray process may be used to produce cermet coatings through particle melting or partial melting, and acceleration of particles onto a substrate. Typical thermal spray coatings are produced by melting of the particles and re-solidification on the substrate. In this process, a feedstock material is heated to an elevated temperature in a spray device and the heated feedstock material is ejected from the spray device at a high velocity and thence sprayed against a substrate article surface. The droplets and particles impact the surface at a high velocity, and are flattened against the surface to form a solidified coating having a desired thickness.
Thermal spray processes often use combustion of fuel with an oxidizer to provide the heat to the feedstock material. Two combustion thermal spray processes, high velocity oxygen fuel (HVOF) and high velocity air fuel (HVAF) techniques, are sometimes used to form coatings. In each technique, a gas or liquid fuel is combusted with oxygen (HVOF) or air (HVAF) to produce a high velocity exhaust stream. A feedstock powder injected into the exhaust stream is heated and accelerated toward the desired substrate at sonic or supersonic speeds.
However, feedstock particles having an average diameter smaller than about 15-20 microns tend to clog or agglomerate in conventional HVOF and HVAF equipment, affecting the feeding rate and the quality of the coating. Further, the HVOF or the HVAF process, by the nature of combustion process, may produce very high combustion temperatures that result in high particle temperatures that could lead to oxidation, decarburization, or dissolution of the ceramic particles in the metallic binder matrix, which may affect the properties of the coatings.
Thus, there is a need for improved methods of depositing cermet coatings using the thermal spray process. Further, there is a need for cost-effective coatings having the desired properties and deposited using the combustion-based thermal spray process.